Under a framework system using the concept of Internet of global energy, half wave transmission attracts more and more attentions as an alternating current long-distance transmission technology. Half wave transmission refers to that a length of a transmission line is a distance of transmission of an electromagnetic wave in a half period (when a frequency is 50 hertz (Hz), the corresponding length is 3,000 kilometers). For a half wavelength transmission line, due to the fact that a length of the line is far more than a conventional transmission distance, its electromagnetic wave presents an obvious fluctuation characteristic, which makes normal operation and fault electrical characteristics of the half wavelength line different from a conventional medium/short-distance transmission line. Conventional line protection may not be adapted to the characteristic and operation requirement of the half wavelength line. In order to implement protection over the half wavelength line, a new protection solution and method are required to be discussed in combination with its fault electrical characteristic.
A free wave is a free component in the fault transient process of a half wavelength transmission system. In case of faults in and outside a half wavelength line region, the free wave represents radically different waveform characteristics. The disclosure discloses a novel half wavelength protection method using such a characteristic. In the existing protection solution, traveling wave protection is applied more widely in direct current transmission line, but it also has some shortcomings. For example: an extremely high sampling rate is required, which is usually hundreds of KHz; the influence of a capacitive voltage transformer exists; and non-unit traveling wave protection is inaccurate. Therefore, it is necessary to seek for a free wave-based protection method capable of overcoming these shortcomings to improve safety of a half wavelength transmission line.